Sector: A sector is a part of a circle.
Chord: A chord is a straight line connecting two points on a circle.
Diameter: A diameter is a straight line passing through the center of the circle and touching two points on the circle.
Diagonal: A diagonal, on the other hand, is not specific to circles; it refers to a line connecting two non-adjacent vertices of a polygon.
Therefore, the correct group consists of 1, 2, and 4.